The ability to grow myeloid and erythroid hematopoietic cells in vitro has provided a convenient method for the assay and enumeration of human and murine stem cells. The dependence of erythropoietic mouse spleen and fetal liver cells on the hormone erythropoietin has served as a basis for several assays of antibodies to erythropoietin produced by hybridoma cells (see project Z01-HL-02309-01). This report describes the usefulness of cell cutlre techniques in the evaluation of patients with pure red cell aplasia and aplastic anemia who are treated with immunotherapy, including the drugs cyclophosphamide and prednisone combine with plasmapheresis or lymphocyte depletion. In addition, the method serves as a basis for assessment of cell separation techniques in the partial purification of peripheral blood stem cells.